xs (list of Variable or N-dimensional array) – Input variables to be concatenated. The variables must have the
same ndim. When the variables have the second axis (i.e.
\(ndim \geq 2\)), the variables must have the same shape
along all but the second axis. When the variables do not have the
second axis(i.e. \(ndim < 2\)), the variables need not to have
the same shape.

Returns

Output variable. When the input variables have the second axis
(i.e. \(ndim \geq 2\)), the shapes of inputs and output are
the same along all but the second axis. The length of second axis
is the sum of the lengths of inputs’ second axis.
When the variables do not have the second axis (i.e.
\(ndim < 2\)), the shape of output is (N,) (N is the
sum of the input variables’ size).